The IPHC requests your help as we design and implement a sampling program to determine the sex of halibut that are landed by the commercial longline fishery. Accurate sex-ratio information is necessary for stock assessment - most notably, for accurately estimating and monitoring spawning stock biomass.
If I’m taking the time and effort to mark all halibut, why doesn’t that information get recorded by the plant or IPHC port sampler?
We rely on random sampling so that the data we collect are representative of the entire catch. Just like our otolith sampling, we cannot add samples outside our normal frame without creating bias. You still get hats though (and maybe a jacket)! An important part of this year’s study is testing the marking procedure. Please provide feedback to your port sampler regarding how it went.
Why do we have to mark all of the catch?
The most informative data comes from the sex-ratio at age, and to get that we need a mark on every fish for which we collect a length, weight, and otolith. If you can guess which of your fish will be selected for otolith sampling, then just mark those.
Why do we have to mark both male and female? If fishermen only marked males, couldn’t you tell the others are females?
We would know that the marked fish were males; but we couldn’t be sure that all of the unmarked fish were females. Mistakes happen, and some fish will surely go unmarked. If only the males were marked, then every “mistake” would become a female even if had been a male. The result would be a sex ratio that is mistakenly biased towards females (or, if only the females were marked, biased towards males). By marking both sexes, the sex ratios estimated from the markings should – on average – be correct even if a small number of fish slip through the process unmarked.
Why is it important if the sex ratio of the commercial catch is different than the survey catch?
The commercial catch is removed from the population, and we need to know its properties to estimate the impact of these removals on the stock. It is also quite likely that the fishery is encountering males and females in different proportions in the spring and fall, when we have no survey data.
If you have an unanswered question, please contact Tim Loher.